Reverse Osmosis Technical Data

The enclosed technical data is taken from the GE Osmonics Pure Water Hand Book and is not represented as being original. The handbook is available in its complete form at: http://www.osmonics.com/library/pwh.htm This data is being provided for your use in understanding the hydrologic cycle and the related water contamination issues and solutions thereof in greater detail than provided elsewhere in this web site. Water purification can be a very complex issue depending on the severity of contamination as you will see in the following discussion. A reverse osmosis purification system such as the Vagabond Reverse osmosis system is a greatly simplified way to purify water sourced from municipal distribution lines where pre treatment has been applied as necessary. The Vagabond System is not intended for use on visibly turbid water as such water will overwhelm the pre filters and restrict water flow. In the event that you want to clean up and purify really gross water, we can help you find a solution for your specific needs. The following discussion will give you an example of the kind of equipment commonly used for mere difficult applications.

Table of Contents1.0 Introduction

2.0 Water The Problem of Purity2.1 Natural Contamination and Purification2.2 Bacterial Contamination

3.0 Identifying Impurities3.1 General Qualitative Identification

TurbidityTasteColorOdorFurther Analysis

3.2 General Quantitative IdentificationpHTotal SolidsConductivity/ResistivityMicrobiological Contamination

3.3 Specific ImpuritiesCommon IonsDissolved GasesHeavy MetalsDissolved Organic CompoundsVolatile Organic Compounds (VOC)Radioactive Constituents

4.0 Methods of Water Purification4.1 Municipal or Utility Water Treatment

Screen PrefiltrationClarificationLime-Soda Ash TreatmentDisinfectionpH Adjustment

4.2 On-Site TreatmentChemical AdditionTank-Type Pressure FiltersPre-Coat FiltersCartridge Filters

Pure Water Handbook-twg 5/16/97 2:20 PM Page 7

Ion Exchange SystemsOrganic ScavengingDistillation and Pure Steam GeneratorsElectrodialysisCrossflow Filtration Systems

(Reverse Osmosis and Similar Processes)Membrane ConfigurationsDisinfection Control of Microbes

5.0 Examples of High-Purity Water Treatment Systems5.1 Potable Water

Residential Water Purification System5.2 Kidney Dialysis

Single-Patient Dialysis15-Bed, In-Center Dialysis System, with Recycle14-Bed, In-Center Dialysis, Continuous Flow Direct Feed

5.3 Commercial-Scale Purified Water Treatment System5.4 Water for Pharmaceutical Use

USP Purified Water SystemUSP Water for Injection System

5.5 Boiler Feed and Power Generator WaterHigh-Pressure Steam Generation

5.6 Potable Water/Boiler Feed/Humidification/General Rinse5.7 Water for Electronics5.8 Water for Laboratory Use

Reagent-Grade Water for Laboratory Use5.9 Water for Beverage Manufacturing

Bottled WaterSoft DrinksJuicesBeverage Water RequirementsBottled Water Requirements


7.0 Appendices(as of February 1996)

7.2 Appendix B: Electronic Grade Water7.3 Appendix C: Reagent-Grade Water7.4 Appendix D: USP 23 WFI and Purified Water

StandardsWater for InjectionPurified Water

7.5 Appendix E: Metric Conversions7.6 Appendix F: Silt Density Index7.7 Appendix G

Langelier Stability Indexes (LSI)Nomograph for Determining Langelier

or Ryznar Indexes7.8 Appendix H

Effect of Bicarbonate Alkalinity and CO2 on pHEffect of Mineral Acidity on pH1Effect of Carbonate and Bicarbonate Alkalinity on pH

7.9 Appendix I: Sieve Mesh Conversion Table

8.0 Glossary of Water Purification Terms


INTRODUCTION

1.0 INTRODUCTION

For more than 30 years there has been remarkable growth in the need for quality water purification by all categories of users municipal, industrial, institutional, medical, commercial and residential. The increasingly broad range of requirements for water quality has motivated the water treatment industry to refine existing techniques, combine methods and explore new water purification technologies.

Although great improvements have been made, myths and misconceptions still exist. This Pure Water Handbook by Osmonics will clear up common misconceptions and increase the readers understanding of the capabilities of available technologies and how these technologies might be applied.

Science has found that there are no two water treatment problems exactly alike. There will always be slight differences with more than one technically -acceptable and scientifically-sound solution to any given water treatment problem. Beyond these two statements, there are no absolutes in water treatment.


WATER THE PROBLEM OF PURITY

2.0 WATER THE PROBLEM OF PURITY

In its pure state, water is one of the most aggressive solvents known. Called the universal solvent, water, to a certain degree, will dissolve virtually everything to which it is exposed. Pure water has a very high energy state and, like everything else in nature, seems to achieve energy equilibrium with its surroundings. It will dissolve the quantity of material available until the solution reaches saturation, the point at which no higher level of solids can be dissolved. Contaminants found in water include atmospheric gases, minerals, organic materials (some naturally-occurring, others man-made) plus any materials used to transport or store water. The hydrologic cycle (Figure 1) illustrates the process of contamination and natural purification.

Figure 1 Hydrologic Cycle

ROCK STRATA(CONFINING LAYER)

GROUND WATER STORAGE

WATER TABLE

OCEAN

LAKE

RIVER

SURFACE RUNOFF

PERCOLATION

EVAPORATION

TRANSPIRATION

EVAPORATIONPRECIPITATION


2.1 Natural Contamination and Purification

Water evaporates from surface supplies and transpires from vegetation directly into the atmosphere.

The evaporated water then condenses in the cooler air on nuclei suchas dust particles and eventually returns to the earths surface as rain,snow, sleet, or other precipitation. It dissolves gases such as carbondioxide, oxygen, and natural and industrial emissions such as nitricand sulfuric oxides, as well as carbon monoxide. Typical rain waterhas a pH of 5 to 6. The result of contact with higher levels of thesedissolved gases is usually a mildly acidic condition what is todaycalled acid rain that may have a pH as low as 4.0.

As the precipitation nears the ground, it picks up many additionalcontaminants - airborne particulates, spores, bacteria, and emis-sions from countless other sources.

Most precipitation falls into the ocean, and some evaporates beforereaching the earths surface. The precipitation that reaches landreplenishes groundwater aquifers and surface water supplies.

The water that percolates down through the porous upper crust ofthe earth is substantially filtered by that process. Most of the particulate matter is removed, much of the organic contamination is consumed by bacterial activity in the soil, and a relatively clean,mildly acidic solution results. This acidic condition allows the waterto dissolve many minerals, especially limestone, which contributescalcium. Other geologic formations contribute minerals, such asmagnesium, iron, sulfates and chlorides. The addition of these minerals usually raises groundwater pH to a range of 7 to 8.5.

This mineral-bearing water is stored in natural underground forma-tions called aquifers. These are the source of the well water used byhomes, industries and municipalities.

Surface waters such as rivers, lakes and reservoirs typically containless mineral contamination because that water did not pass throughthe earths soils. Surface waters will, however, hold higher levels oforganics and undissolved particles because the water has contactedvegetation and caused runoff to pick up surface debris.



2.2 Bacterial Contamination

One difficulty of water purity is bacterial contamination and controlof bacterial growth.

Water is essential for all life. It is a necessary medium for bacterialgrowth because it carries nutrients. It is an essential component ofliving cells. Its thermal stability provides a controlled environment.Water will support bacterial growth with even the most minute nutrient sources available.



3.0 IDENTIFYING IMPURITIES

The impact of the various impurities generated during the hydrologic cycleand/or bacterial colonization depends upon the water users particularrequirements. In order to assess the need for treatment and the appropriatetechnology, the specific contaminants must be identified and measured.

3.1 General Qualitative IdentificationQualitative identification is usually used to describe the visible oraesthetic characteristics of water. Among others these include:

turbidity (clarity) taste color odor

TurbidityTurbidity consists of suspended material in water, causing a cloudyappearance. This cloudy appearance is caused by the scattering andabsorption of light by these particles. The suspended matter may be inorganic or organic. Generally the small size of the particles prevents rapid settling of the material and the water must be treatedto reduce its turbidity.

Correlation of turbidity with the concentration of particles present isdifficult since the light-scattering properties vary among materialsand are not necessarily proportional to their concentration.

Turbidity can be measured by different optical systems. Such measurements simply show the relative resistance to light transmit-tance, not an absolute level of contamination.

A candle turbidimeter is a very basic visual method used to measure highly turbid water. Its results are expressed in JacksonTurbidity Units (JTU). A nephelometer is more useful in low-turbidity water, with results expressed in Nephelometric TurbidityUnits (NTU) or Formazin Turbidity Units (FTU). JTU and NTU arenot equivalent.

Suspended matter can also be expressed quantitatively in parts permillion (ppm) by weight or milligrams per liter (mg/L). This isaccomplished by gravimetric analysis, typically filtering the sample

IDENTIFYING IMPURITIES


through a 0.45-micron membrane disc, then drying and weighingthe residue.

The Silt Density Index (SDI) provides a relative value of suspendedmatter. The measured values reflect the rate at which a 0.45-micronfilter will plug with particulate material in the source water. The SDItest is commonly used to correlate the level of suspended solids infeedwater that tends to foul reverse osmosis systems.

TasteThe taste sense is moderately accurate and able to detect concentra-tions from a few tenths to several hundred ppm. However, tasteoften cannot identify particular contaminants. A bad taste may be an indication of harmful contamination in drinking water, but certainlycannot be relied on to detect all harmful contaminants.

ColorColor is contributed primarily by organic material, although somemetal ions may also tint water. While not typically a health concern,color does indicate a certain level of impurities, and can be an aesthetic concern. True color refers to the color of a sample withits turbidity removed. Turbidity contributes to apparent color.Color can be measured by visual comparison of samples with calibrated glass ampules or known concentrations of colored solutions. Color can also be measured using a spectrophotometer.

OdorThe human nose is the most sensitive odor-detecting device available. It can detect odors in low concentrations down to partsper billion (ppb). Smell is useful because it provides an early indi-cation of contamination which could be hazardous or at least reducethe aesthetic quality of the water.

Further AnalysisFurther analysis should focus on identification and quantification ofspecific contaminants responsible for the water quality. Such conta-minants can be divided into two groups: dissolved contaminants andparticulate matter. Dissolved contaminants are mostly ionic atoms ora group of atoms carrying an electric charge. They are usually associated with water quality and health concerns. Particulate matter typically silt, sand, virus, bacteria or color-causing particles isnot dissolved in water. Particulate matter is usually responsible




for aesthetic characteristics such as color, or parameters such as turbidity, which affects water processes.

3.2 General Quantitative IdentificationFollowing are the major quantitative analyses which define waterquality.

pHThe relative acidic or basic level of a solution is measured by pH.The pH is a measure of hydrogen ion concentration in water, specifi-cally the negative logarithm (log) of the hydrogen ion concentration.The measurement of pH lies on a scale of 0 to 14 (Figure 2), with apH of 7.0 being neutral (i.e., neither acidic nor basic), and bearingequal numbers of hydroxyl (OH-) and hydrogen (H+) ions. A pH ofless than 7.0 is acidic; a pH of more than 7.0 is basic.

pH0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

more acidic neutral more basic

Figure 2 pH Value

Since pH is expressed in log form, a pH of 6.0 is 10 times moreacidic than a pH of 7.0, and a pH of 5.0 is 100 times more acidicthan a pH of 7.0. The pH has an effect on many phases of watertreatment such as coagulation, chlorination and water softening. Italso affects the scaling-potential of water sources.

The pH level can be determined by various means such as colorindicators, pH paper or pH meters. A pH meter is the most commonand accurate means used to measure pH.


Total SolidsTotal Solids (TS) (Table 1) is the sum of Total Dissolved Solids(TDS) and Total Suspended Solids (TSS). In water analysis thesequantities are determined gravimetrically by drying a sample andweighing the residue. In the field, TDS is commonly measured by a conductivity meter (Figure 3) which is correlative to a specific saltsolution; however, this measurement is only an approximation mostoften based on a multiplication factor of 0.66 of the electrical conductivity. (See Table 2 page 20.)

Table 1 Example Total Solids (TS)

TDS TSSOrganic Inorganic Organic Inorganichumic acid reactive silica algae silttannin (dissolved) fungi rustpyrogens salt ions bacteria floc

clays

Conductivity/ResistivityIons conduct electricity. Because pure water contains few ions, it has a high resistance to electrical current. The measurement ofwaters electrical conductivity, or resistivity, can provide an assess-ment of total ionic concentration. Conductivity is described inmicroSiemens/cm (S) and is measured by a conductivity meter(Figure 4) and cell. Resistivity is described in megohm-cm, is theinverse of conductivity and is measured by a resistivity meter and cell.




Figure 3 Field Conductivity Meter

Figure 4 On-Line Conductivity Meter


Table 2 expresses the relative concentrations of sodium chloride versus conductivity and resistance. As a general rule, ionic-dissolvedcontent, expressed in ppm or mg/L, is approximately one-half totwo-thirds the conductance of water. Other salt solutions are usedand the curve varies. Monovalent salts have higher conductivitiesthan multivalent salts.

Table 2 Relative Concentration of Dissolved Minerals versus Conductivity and Resistance @ 25C

mg/L Total Dissolved Specific Specific GrainsSodium Solids Conductance Resistance perChloride mg/L CaCO3 MicroSiemens/cm ohms/cm Gallon

0.05 0.043 0.105 9,523,800 0.00250.1 0.085 0.212 4,716,980 0.00490.2 0.170 0.424 3,558,490 0.00990.3 0.255 0.637 1,569,850 0.01490.4 0.340 0.848 1,179,240 0.01980.5 0.425 1.06 943,396 0.02480.6 0.510 1.273 785,545 0.02980.7 0.595 1.985 673,400 0.03470.8 0.680 1.696 589,622 0.03970.9 0.765 1.908 524,109 0.0447

1.0 0.85 2.12 471,698 0.04972.0 1.70 6.37 156,985 0.09944.0 3.40 8.48 117,924 0.19885.0 4.25 10.6 94,339 0.24856.0 5.10 12.73 78,554 0.29827.0 5.95 14.85 67,340 0.34798.0 6.80 16.96 58,962 0.39769.0 7.65 19.08 52,410 0.4473

10.0 8.5 21.2 47,169 0.497020.0 17.0 42.4 23,584 0.994130.0 25.5 63.7 15,698 1.491240.0 34.0 84.8 11,792 1.988350.0 42.5 106.0 9,433 2.485360.0 51.0 127.3 7,855 2.982470.0 59.5 148.5 6,734 3.479580.0 68.0 169.6 5,896 3.976690.0 76.5 190.8 5,241 4.4736

continued



100.0 85.0 212.0 4,716 4.9707200.0 170.0 410.0 2,439 9.9415300.0 255.0 610.0 1,639 14.9122400.0 340.0 812.0 1,231 19.8830500.0 425.0 1,008.0 992 24.8538600.0 510.0 1,206.0 829 29.8245700.0 595.0 1,410.0 709 34.7953800.0 680.0 1,605.0 623 39.7660900.0 765.0 1,806.0 553 44.7368

1,000.0 850.0 2,000.0 500 49.70762,000.0 1,700.0 3,830.0 261 99.41523,000.0 2,550.0 5,670.0 176 149.12284,000.0 3,400.0 7,500.0 133 198.83045,000.0 4,250.0 9,240.0 108 248.53806,000.0 5,100.0 10,950.0 91 298.24567,000.0 5,950.0 12,650.0 79 347.95328,000.0 6,800.0 14,340.0 69 397.66089,000.0 7,650.0 16,000.0 62 447.3684

10,000.0 8,500.0 17,600.0 56 497.0760

Microbiological ContaminationMicrobiological contamination can be classified as viable and nonvi-able. Viable organisms are those that have the ability to reproduceand proliferate. Nonviable organisms cannot reproduce or multiply.

Bacterial ContaminationBacterial contamination is quantified as Colony Forming Units(CFU), a measure of the total viable bacterial population. CFUs aretypically determined by incubating a sample on a nutritional mediumand counting the number of bacterial colonies that grow. Eachcolony is assumed to have grown from a single bacterial cell. This iscalled a Standard Plate Count and is the most common method.Other less common methods of enumerating microbial contamina-tion include the Most Probable Number, which is a statistical probability of the bacterial population in a small sample, and theDirect Count, which is an actual count of cells observed through a microscope.

Pyrogenic ContaminationPyrogens are substances that can induce a fever in a warm-bloodedanimal. The most common pyrogenic substance is the bacterialendotoxin. These endotoxins are lipopolysaccharide compounds



from the cell walls of gram-negative bacteria. They can be pyrogenicwhether they are part of intact viable cells or simply fragments fromruptured cells.They are more stable than bacterial cells and are notdestroyed by all conditions (such as autoclaving) that kill bacteria.Their molecular weight (MW) is generally accepted to be approxi-mately 10,000. One molecular weight (MW) is approximately equalto one dalton. However, in aqueous environments they tend toagglomerate to larger sizes. Pyrogens are quantified as EndotoxinUnits per milliliter (EU/mL).

The traditional method for pyrogen detection used live rabbits as the test organism. Today the most common method is the LimulusAmoebocyte Lysate (LAL) test. Endotoxins react with a purifiedextract of the blood of the horseshoe crab Limulus polyphemus andthis reaction can be used to determine the endotoxin concentration.There are several versions of the LAL test ranging from the semi-quantitative gel-clot method to the fully-automated kineticturbidmetric method which is sensitive to 0.001 EU/mL. There isan endotoxin limit in the pharmaceutical industry for USP WaterFor Injection (WFI) of 0.25 EU/mL. The LAL test is relativelyquick and inexpensive.

The LAL test is used if there is a concern about endotoxins in thefinished water, such as in pharmaceutical uses. However, due to the swift results and the relatively low cost of the LAL test, otherindustries with critical water quality needs are beginning to use it asa quick indicator of possible bacterial contamination or total organiccarbon (TOC).

Total Organic Carbon (TOC)TOC is a direct measure of the organic, oxidizable, carbon-basedmaterial in water. TOC is a vital measurement used in sophisticatedwater treatment systems such as electronics grade where anyamount of contamination can adversely affect product quality andyield.

Biochemical Oxygen Demand (BOD)BOD is a measure of organic material contamination in water, specified in mg/L. BOD is the amount of dissolved oxygen requiredfor the biochemical decomposition of organic compounds and theoxidation of certain inorganic materials (e.g., iron, sulfites).Typically the test for BOD is conducted over a five-day period.



Chemical Oxygen Demand (COD) COD is another measure of organic material contamination in waterspecified in mg/L. COD is the amount of dissolved oxygen requiredto cause chemical oxidation of the organic material in water.

Both BOD and COD are key indicators of the environmental healthof a surface water supply. They are commonly used in waste watertreatment but rarely in general water treatment.

3.3 Specific Impurities

Many individual impurities can be quantified through water analysis techniques. Below is a discussion of most ionic individualcontaminants.

Common IonsA number of terms are used to express the level of mineral contamination in a water supply.

Table 3 Units of Concentration

Unit Abbreviation Describesmilligrams per liter mg/L (weight per volume)parts per million ppm (weight in weight)parts per billion ppb (weight in weight)parts per trillion ppt (weight in weight)grains per gallon gpg (weight per volume)milli-equivalents per liter m eq/L (weight per volume)

A conversion table (Table 4) illustrates the relationships.

Table 4 Conversions

mg/L /17.1 = gpgppm /17.1 = gpggpg x 17.1 = ppm or mg/Lmg/L (expressed as CaCO3) x 50 = m eq/Lppm x 1000 = ppbppb x 1000 = ppt



Water HardnessThe presence of calcium (Ca2+) and magnesium (Mg2+) ions in a water supply is commonly known as hardness. It is usuallyexpressed as grains per gallon (gpg). Hardness minerals exist tosome degree in virtually every water supply. The following tableclassifies the degree of hardness:

Table 5 Water Hardness Classification

Hardness Level Classificationmg/L gpg0-17 180 >10.5 very hard water

The main problem associated with hardness is scale formation. Even levels as low as 5 to 8 mg/L (0.3 to 0.5 gpg) are too extremefor many uses. The source of hardness is calcium- and magnesium-bearing minerals dissolved in groundwater. Carbonate and noncarbonate hardness are terms used to describe the source of calcium and magnesium. Carbonate hardness usually resultsfrom dolomitic limestone (calcium and magnesium carbonate) whilenoncarbonate hardness generally comes from chloride and sulfatesalts.

IronIron, which makes up 5% of the earths crust, is a common watercontaminant. It can be difficult to remove because it may changevalence states that is, change from the water-soluble ferrous state(Fe2+) to the insoluble ferric state (Fe3+). When oxygen or an oxidizing agent is introduced, ferrous iron becomes ferric which is insoluble and so precipitates, leading to a rusty (red-brown)appearance in water. This change can occur when deep well water is pumped into a distribution system where it adsorbs oxygen. Ferric iron can create havoc with valves, piping, water treatmentequipment, and water-using devices.

Certain bacteria can further complicate iron problems. Organismssuch as Crenothrix, Sphaerotilus and Gallionella use iron as an energy source. These iron-reducing bacteria eventually form a rusty,gelatinous sludge that can plug a water pipe. When diagnosing an




iron problem, it is very important to determine whether or not suchbacteria are present.

ManganeseAlthough manganese behaves like iron, much lower concentrationscan cause water system problems. However, manganese does notoccur as frequently as iron. Manganese forms a dark, almost black,precipitate.

SulfateSulfate (SO42-) is very common. When present at lower levels, sulfate salts create problems only for critical manufacturing processes. At higher levels, they are associated with a bitter taste and laxative effect. Many divalent metal-sulfate salts are virtuallyinsoluble and precipitate at low concentrations.

ChlorideChloride (Cl-) salts are common water contaminants. The criticallevel of chloride depends on the intended use of the water. At highlevels, chloride causes a salty or brackish taste and can interferewith certain water treatment methods. Chlorides also corrode themetals of water supply systems, including some stainless steels.

AlkalinityAlkalinity is a generic term used to describe carbonates (CO32-),bicarbonates (HCO3-) and hydroxides (OH-). When present withhardness or certain heavy metals, alkalinity contributes to scaling.The presence of alkalinity may also raise the pH.

Nitrate - NitriteAlthough nitrate (NO3-) and nitrite (NO2-) salts may occur naturally,their presence in a water supply usually indicates man-made pollution. The most common sources of nitrate/nitrite contamina-tion are animal wastes, primary or secondary sewage, industrial chemicals and fertilizers. Even low nitrate levels are toxic tohumans, especially infants, and contribute to the loss of young livestock on farms with nitrate-contaminated water supplies.

ChlorineChlorine, because of its bactericidal qualities, is important in thetreatment of most municipal water supplies. It is usually monitoredas free chlorine (Cl2) in concentrations of 0.1 to 2.0 ppm. In solu-tion, chlorine gas dissolves and reacts with water to form the


hypochlorite anion (ClO-) and hypochlorous acid (HClO). The rela-tive concentration of each ion is dependent upon pH. At a neutral pHof 7, essentially all chlorine exists as the hypochlorite anion which isthe stronger oxidizing form. Below a pH of 7, hypochlorous acid isdominant, and has better disinfectant properties than the anion counter-part. Although chlorines microbial action is generally required, chlorine and the compounds it forms may cause a disagreeable taste and odor. Chlorine also forms small amounts of trihalogenatedmethane compounds (THMs), which are a recognized health hazardconcern as carcinogenic materials. The organic materials withwhich the chlorine reacts are known as THM precursors.

ChloraminesIn some cases, chlorine is also present as chloramine (i.e., monochloramine, NH2Cl) as a result of free chlorine reacting withammonia compounds. The ammonia is added to a water supply tostabilize the free chlorine. Chloramines are not as effective amicrobial deterrent as chlorine, but provide longer-lasting residuals.

Chlorine DioxideThis material is often produced on-site primarily by large municipali-ties via the reaction between chlorine or sodium hypochlorite andsodium chlorite. A more costly source of chlorine dioxide is availableas a stabilized sodium chlorite solution. Chlorine dioxide has beenused for taste and odor control and as an efficient biocide. Chlorinedioxide can maintain a residual for extended periods of time in a distribution system and does not form trihalomethanes (THMs) orchloramines if the stabilized sodium chlorite form is used. The possible toxicity of the chlorate and chlorite ions (reaction by-products) may be a concern for potable water applications.

SilicaEvery water supply contains at least some silica (SiO2). Silica occursnaturally at levels ranging from a few ppm to more than 200 ppm. It is one of the most prevalent elements in the world. Among theproblems created by silica are scaling or glassing in boilers, stills,and cooling water systems, or deposits on turbine blades. Silica scaleis difficult to remove.

Silica chemistry is complex. An unusual characteristic of silica is itssolubility. Unlike many scaling salts, silica is more soluble at higherpH ranges. Silica is usually encountered in two forms: ionic and colloidal (reactive and nonreactive based on the typical analytical



techniques). Silica can be present in natural waters in a combinationof three forms: reactive (ionic), nonreactive (colloidal) and particulate.

Ionic Silica (reactive)Ionic or reactive silica exists in an SiO2 complex. It is not a strongly-charged ion and therefore is not easily removed by ion exchange. However, when concentrated to levels above 100 ppm, ionic silica may polymerize to form a colloid.

Colloidal Silica (nonreactive)At concentrations over 100 ppm, silica may form colloids of 20,000 molecular weight and larger, still too small to be effectively removed by a particle filter. Colloidal silica is easily removed with ultrafiltration, or can be reduced by chemical treatment (lime softening).

Colloidal silica can lower the efficiency of filtration systems (suchas reverse osmosis). Any silica can affect yields in semiconductormanufacturing and is a major concern in high-pressure boiler systems.

AluminumAluminum (Al3+) may be present as a result of the addition of aluminum sulfate [Al2(SO4)3] known as alum, a commonly usedflocculant. Aluminum can cause scaling in cooling and boiler systems, is a problem for dialysis patients, and may have someeffects on general human health. Aluminum is least soluble at theneutral pH common to many natural water sources.

SodiumThe sodium ion (Na+) is introduced naturally due to the dissolutionof salts such as sodium chloride (NaCl), sodium carbonate(Na2CO3), sodium nitrate (NaNO3) and sodium sulfate (Na2SO4). It is also added during water softening or discharge from industrialbrine processes. By itself the sodium ion is rarely a problem, butwhen its salts are the source of chlorides (Cl-) or hydroxides (OH-),it can cause corrosion of boilers, and at high concentrations (such asseawater) it will corrode stainless steels.

PotassiumPotassium is an essential element most often found with chloride(KCl) and has similar effects but is less common than sodium chlo-ride. It is used in some industrial processes. The presence of KCl is



typically a problem when only ultrapure water quality is required.

PhosphateMost phosphates (PO43-) commonly enter surface water suppliesthrough runoff of fertilizers and detergents in which phosphatesare common ingredients. Phosphates also enter the hydrologic cyclethrough the breakdown of organic debris.

Phosphates are used in many antiscalant formulations. At the levelsfound in most water supplies phosphates do not cause a problemunless ultrapure water is required. Phosphates may foster algaeblooms in surface waters or open storage tanks.

Dissolved Gases Carbon DioxideDissolved carbon dioxide (CO2) associates with water molecules to form carbonic acid (H2CO3), reducing the pH and contributing to corrosion in water lines, especially steam and condensate lines.Carbonic acid, in turn, dissociates to bicarbonate (HCO3-) or carbonate (CO32-), depending on pH. Most of the CO2 found inwater comes not from the atmosphere but from carbonate that thewater has dissolved from rock formations.

OxygenDissolved oxygen (02) can corrode water lines, boilers and heatexchangers, but is only soluble to about 14 ppm at atmospheric pressure.

Hydrogen SulfideThe infamous rotten egg odor, hydrogen sulfide (H2S) can contribute to corrosion. It is found primarily in well water suppliesor other anaerobic sources. H2S can be readily oxidized by chlorineor ozone to eliminate sulfur.

RadonRadon is a water-soluble gas produced by the decay of radium andits isotopes. It is the heaviest gas known and occurs naturally ingroundwater from contact with granite formations, phosphate anduranium deposits. Prolonged exposure may cause human healthproblems including cancer.

Heavy MetalsHeavy metals such as lead, arsenic, cadmium, selenium and



chromium when present above certain levels can have harmfuleffects on human health. In addition, minute concentrations mayinterfere with the manufacture and effectiveness of pharmaceuticalproducts, as well as laboratory and industrial processes of a sensitivenature.

Dissolved Organic CompoundsDissolved organic materials occur in water both as the product ofmaterial decomposition and as pollution from synthetic compoundssuch as pesticides.

Naturally-OccurringTannins, humic acid and fulvic acids are common natural contaminants. They cause color in the water and detract from theaesthetics of water but, unless they react with certain halogens, theyhave no known health consequences in normal concentrations. In the presence of free halogen compounds (principally chlorine orbromine), they form chlorinated hydrocarbons and trihalomethanes(THMs), which are suspected carcinogens. Maximum allowable limits of THMs in municipal systems have been imposed by theUnited States Environmental Protection Agency (EPA).

Synthetic Organic Compounds (SOCs)A wide variety of synthetic compounds which are potential healthhazards are present in water systems due to the use of industrial andagricultural chemicals. These compounds are not readily biodegrad-able and leach from soil or are carried by runoff into water sources.Many are suspected carcinogens and are regulated by the EPA.

Volatile Organic Compounds (VOC)Due to relatively low molecular weight, many synthetic organic compounds such as carbon tetrachloride, chloroform and methylenechloride will easily volatilize. Volatility is the tendency of a compound to pass into the vapor state. Most are introduced into thewater supply in their liquid phase. If ingested they may be absorbedinto the bloodstream. Many are suspected carcinogens.

Radioactive ConstituentsWater in itself is not radioactive but may contain radionuclides. Theyare introduced either as naturally-occurring isotopes (very rare) orrefined nuclear products from industrial or medical processes,radioactive fallout or nuclear power plants.



4.0 METHODS OF WATER PURIFICATION

Water treatment can be defined as any procedure or method used to alter the composition or behavior of a water supply. Water supplies are classified as either surface water or groundwater. This classification often determines the condition and therefore the treatment of the water. The majority of public or municipal water comes from surface water such as rivers, lakes and reservoirs. The majority of private water supplies consist of groundwater pumped from wells.

4.1 Municipal or Utility Water Treatment

Most municipal water distributed in a city or community today hasbeen treated extensively. Specific water treatment methods and stepstaken by municipalities to meet local, state or national standardsvary, but are categorized below.

Screen PrefiltrationA coarse screen, usually 50 to 100 mesh (305 to 140 microns), at the intake point of a surface water supply, removes large particulatematter to protect downstream equipment from clogging, fouling, orphysical damage.

ClarificationClarification (Figure 6) is generally a multistep process to reduceturbidity and remove suspended matter. First, the addition of chemical coagulants or pH-adjustment chemicals react to form floc.The floc settles by gravity in settling tanks or is removed as thewater percolates through a gravity filter. The clarification processeffectively removes particles larger than 25 microns. Clarificationsteps may also be taken to reduce naturally-occurring iron, and toremove colors, taste, and odor by adding strong oxidizing agentssuch as chlorine. Where gravity filters are used, activated carbonslurries are sometimes added to aid in color and odor removal.

Clarification can remove a high percentage of suspended solids at arelatively low cost per gallon. However, most clarification processeswill not remove all types of suspended or colloidal contamination andremove few dissolved solids. The clarification process is not 100%efficient; therefore, water treated through clarification may still containsome suspended materials.

METHODS OF WATER PURIFICATION


Lime-Soda Ash Treatment The addition of lime (CaO) and soda ash (Na2CO3) reduces the levelof calcium and magnesium and is referred to as lime softening.The purpose of lime softening is to precipitate calcium and magnesium hydroxides (hardness) and to help clarify the water. The process is inexpensive but only marginally effective, usuallyproducing water of 50 to 120 ppm (3 to 7 gpg) hardness. A short-coming of this process is the high pH of the treated water, usually in the 8.5 to 10.0 range. Unless the pH is buffered to approximately7.5 to 8.0, the condition of the water is usually unacceptable for general process use.

Figure 6 Clarifier

DisinfectionDisinfection is one of the most important steps in municipal watertreatment. Usually chlorine gas is fed into the supply after the waterhas been clarified and/or softened. The chlorine kills bacteria. Inorder to maintain the kill potential an excess of chlorine is fed intothe supply to maintain a residual. The chlorine level at outlying


Pure Water Handbook-twg 6/5/97 9:31 AM Page 31


distribution points is usually monitored at a target level of about 0.2 to 0.5 ppm. However, if the water supply is heavily contaminat-ed with organics, the chlorine may react to form chloramines and certain chlorinated hydrocarbons (THMs), many of which are considered carcinogenic. In other cases the chlorine can dissipateand no residual level is maintained at the point-of-use, allowingmicrobial growth to occur. To prevent this problem, some munici-palities add ammonia or other nitrogen compounds to create chloramines. The NH2Cl compounds formed have a much longerhalf-life, allowing a measurable chlorine residual to be maintained to extreme points-of-use. The residual chloramines may pose theirown problems.

pH AdjustmentMunicipal waters may be pH-adjusted to approximately 7.5 to 8.0 toprevent corrosion of water pipes and fixtures, particularly to preventdissolution of lead into a potable water supply. In the case of exces-sive alkalinity, the pH may be reduced by the addition of acid. Thealkalinity will convert to CO2.

4.2 On-Site Treatment

After the water is delivered from the utility or the well, there aremany on-site options for further treatment to meet specific end-userequirements.

Chemical Addition pH AdjustmentCertain chemicals, membranes, ion exchange resins and othermaterials are sensitive to specific pH conditions. For example, prevention of acid corrosion in boiler feedwater typically requirespH adjustment in the range of 8.3 to 9.0.

To raise pH, soda ash or caustic soda may be inexpensively added.However, both cause handling difficulties, require fine-tuning, andadd to the TDS.

To reduce pH, a buffering solution such as sulfuric acid (H2SO4) isadded into the flow with a chemically-resistant pump (Figure 7).



Figure 7 Chemically-Resistant Pumps

Pure Water Handbook-twg 6/5/97 9:16 AM Page 33

DispersantsDispersants, also known as antiscalants, are added when scaling maybe expected due to the concentration of specific ions in the streamexceeding their solubility limit. Dispersants disrupt crystal forma-tion, thereby preventing their growth and subsequent precipitation.

Sequestering (Chelating) AgentsSequestering agents are used to prevent the negative effects of hardness caused by the deposition of Ca, Mg, Fe, Mn and Al.

Oxidizing AgentsOxidizing agents have two distinct functions: as a biocide, or to neutralize reducing agents. For information on biocides, see the section on disinfection.

Potassium PermanganatePotassium permanganate (KMnO4) is a strong oxidizing agent usedin many bleaching applications. It will oxidize most organic compounds and is often used to oxidize iron from the ferrous to the ferric form for ferric precipitation and filtration.

Reducing AgentsReducing agents, like sodium metabisulfite (Na2S2O5), are added toneutralize oxidizing agents such as chlorine or ozone. In membraneand ion exchange systems, reducing agents help prevent the degrada-tion of membranes or resins sensitive to oxidizing agents. Reducingagents are metered into solution and allowed enough residence timefor chemical neutralization. Maintenance of a residual continues toeliminate the oxidizing agent.

Tank-Type Pressure FiltersThere are several types of so-called pressure filters available, eachperforming a specialized task. A single description of the equipmentmechanics is sufficient to understand the principal.

A typical filter consists of a tank, the filter media, and valves or acontroller to direct the filter through its various cycles typicallyservice, backwash and rinse.

Easily the most critical aspect of pressure filter performance is therelationship of flow rates to filter bed area and bed depth. This relationship is the primary cause of trouble and poor performance infilter systems. If problems develop, the most common reason is that



many filters are inaccurately sized for the job. The nominal flowrate in the service cycle depends on bed area available and generallyshould not exceed a nominal rate of 5 gallons (18.8 L) per minute(gpm) per square foot of bed area (12.15 m3/h per m2), with at leasta 30-inch (76.2 cm) filter bed depth.

Another important design criterion is backwash flow rate. Backwashflow rates are a function of backwash water temperature, type, size,and density of media, and the specific design of the pressure filter.Media with densities of 90-100 lb/ft3 generally require 12 to 16 gpm/ft2of bed area. Less dense media may use lower backwash rates. Verycold water uses somewhat lower backwash rates, and warmer waterrequires higher rates. The table below expresses this relationship as afunction of tank diameter. There are many types of filter media butall of them should follow the flow rate guidelines in Table 6.

Table 6 Pressure Filter Size Chart

Tank Bed Maximum MinimumDiameter Area Service Flow Backwash Flow

inch (mm) ft2 (cm2) gpm (m3/h) gpm (m3/h)

8 (203) 0.35 (325) 1.7 (0.4) 2.8 (0.6)10 (254) 0.55 (511) 2.7 (0.6) 4.4 (1.0)13 (330) 0.92 (855) 4.6 (1.0) 7.4 (1.7)16 (406) 1.4 (1301) 7.0 (1.6) 11.2 (2.5)20 (508) 2.2 (2044) 10.9 (2.5) 17.6 (4.0)30 (762) 4.9 (4552) 24.5 (5.6) 39.2 (8.9)42 (1067) 9.6 (8918) 48.0 (10.9) 76.8 (17.4)

NOTE: Minimum backwash flow rates may be higher for some dense media or warmer water [over 77F (25C)].

Some examples of pressure filters and their applications are:

Sand FiltersSand is one filtration medium used to remove turbidity. Sand filterscan economically process large volumes, but have two limitations.The finer sand medium is located on top of coarser support media,which causes the filter to plug quickly and requires frequent back-washing. Also, the coarseness of sand media allows smaller suspended solids to pass, so secondary filters with tighter media are often required.




Neutralizing FiltersNeutralizing filters usually consist of a calcium carbonate, calcitemedium (crushed marble) to neutralize the acidity in low pH water.

Oxidizing FiltersOxidizing filters use a medium treated with oxides of manganese asa source of oxygen to oxidize a number of contaminants includingiron, manganese and hydrogen sulfide. The oxidized contaminantsform a precipitate that is captured by the particle filtration capacityof the medium.

Activated Carbon FiltersActivated carbon (AC) is similar to ion exchange resin in densityand porosity. It adsorbs many dissolved organics and eliminateschlorine or other halogens in water. It does not remove salts. AC filters are one of the only low-cost methods available to remove low-molecular weight (

common for swimming pools, beverage plants, and certain industrialapplications.

Figure 8 Pre-Coat Filter

Cartridge FiltersCartridge filters were once considered only as a point-of-use water treatment method for removal of larger particles. However,breakthroughs in filter design, such as the controlled use of blown microfiber filters (as opposed to wrapped fabric or yarn-wound filters), have tremendously broadened cartridge filter utilization.Cartridge filters fall into two categories: depth filters or surface filters.

Depth Cartridge FiltersIn a depth cartridge filter the water flows through the thick wall ofthe filter where the particles are trapped throughout the complexopenings in the medium. The filter may be constructed of cotton,cellulose and synthetic yarns, chopped fibers bound by adhesives, orblown microfibers of polymers such as polypropylene.

The most important factor in determining the effectiveness of depthfilters is the design of the porosity throughout the thick wall. Thebest depth filters for many applications have lower density on theoutside and progressively higher density toward the inside wall. Theeffect of this graded density (Figure 9) is to trap coarser particlestoward the outside of the wall and the finer particles toward theinner wall. Graded-density filters have a higher dirt-holding capacityand longer effective filter life than depth filters with constant densityconstruction.



Disposal of spent cartridges is an environmental concern; however, somecartridges have the advantage of being easily incinerated.

Figure 9 Depth vs. Surface Media



Depth cartridge filters (Figure 10) are usually disposable and cost-effective, and are available in the particle-removal size range of 0.5 to 100 microns. Generally, they are not an absolute method of filtration since a small amount of particles within the micronrange may pass into the filtrate. However, there are an increasingnumber of depth filters in the marketplace that feature near-absoluteretention ratings.

Figure 10 Microfiber Depth Cartridge Filters



Surface Filtration Pleated Cartridge Filters Pleated cartridge filters (Figure 11) typically act as surface filters.Flat sheet media, either membranes or nonwoven fabric materials,trap particles on the surface. The media are pleated to increaseusable surface area. Pleated filters are usually not cost-effective forwater filtration, where particles greater than one micron quicklyplug them. However, pleated membrane filters serve well as submicron particle or bacteria filters in the 0.1- to 1.0-micron rangeand are often used to polish water after depth filters and other treatment steps in critical applications. Pleated filters are usually dis-posable by incineration, since they are constructed with polymeric materials, including the membrane. Newer cartridges alsoperform in the ultrafiltration range: 0.005- to 0.15-micron.

Figure 11 Pleated Filters (Surface Filtration)




Ultrafiltration (UF) Cartridge FiltersUF membrane cartridges (Figure 12) perform much finer filtrationthan depth filters but are more expensive and require replacement asthe filter becomes blinded, i.e., covered with an impervious thincoating of solids. Typically the smaller the pore the more quicklythis blinding occurs. To avoid blinding of the pores, point-of-useultrafiltration cartridges are built in a spiral-wound configuration toallow crossflow mode operation to help keep the surface clean byrinsing away the solids.

Point-of-use ultrafiltration cartridges are used to remove colloids,pyrogens and other macromolecular compounds from ultrapurewater.

Figure 12 Point-of-Use System


Ion Exchange SystemsAn ion exchange system consists of a tank containing small beads of synthetic resin (Figure 13). The beads are treated to selectivelyadsorb either cations or anions and release (exchange) counter-ions based on the relative activity compared to the resin. This process of ion exchange will continue until all availableexchange sites are filled, at which point the resin is exhausted andmust be regenerated by suitable chemicals.

Ion exchange systems are used in several ways.

Figure 13 Representation of Ion Exchange Resin Bead

Water SofteningThe ion exchange water softener (Figure 14) is one of the most common tools of water treatment. Its function is to remove scale-forming calcium and magnesium ions from hard water. In manycases soluble iron (ferrous) can also be removed with softeners. Astandard water softener has four major components: a resin tank,resin, a brine tank to hold sodium chloride, and a valve or controller.



Figure 14 Duplex Water Softening Resin Tanks

The softener resin tank contains the treated ion exchange resin small beads of polystyrene. The resin bead exchange sites adsorbsodium ions and displace multivalent cations during regenerationwith 6-10% solution of NaCl. The resin has a greater affinity formultivalent ions such as calcium and magnesium than it does forsodium. Thus, when hard water is passed through the resin tank inservice, calcium and magnesium ions adhere to the resin, releasingthe sodium ions until equilibrium is reached.

When most of the sodium ions have been replaced by hardness ions,the resin is exhausted and must be regenerated. Regeneration isachieved by passing a concentrated NaCl solution through the resintanks, replacing the hardness ions with sodium ions. The resins



affinity for the hardness ions is overcome by the concentrated NaClsolution. The regeneration process can be repeated indefinitely without damaging the resin.

Water softening is a simple, well-documented ion exchange process.It solves a very common form of water contamination: hardness.Regeneration with sodium chloride is a simple, inexpensive processand can be automatic, with no strong chemicals required.

The limitations of water softening become apparent when high-quality water is required. Softening merely exchanges the hardnessions for normally less-troublesome sodium ions which are stillunsuitable for many uses.

Deionization (DI)Ion exchange deionizers use synthetic resins (Figure 15) similar to those in water softeners. Typically used on water that has been prefiltered, DI uses a two-stage process to remove virtually all ionicmaterial in water. Two types of synthetic resins are used: one toexchange positively-charged ions (cations) for H+ and another toexchange negatively-charged ions (anions) for OH-.

Cation deionization resins (hydrogen cycle) release hydrogen (H+)in exchange for cations such as calcium, magnesium and sodium.Anion deionization resins (hydroxide cycle) exchange hydroxide(OH-) ions for anions such as chloride, sulfate and bicarbonate. The displaced H+ and OH- combine to form H2O.

Figure 15 Ion-Exchange Resin



Resins have limited capacities and must be regenerated uponexhaustion. This occurs when equilibrium between the adsorbedions is reached. Cation resins are regenerated by treatment with acidwhich replenishes the adsorption sites with H+ ions. Anion resins areregenerated with a base which replenishes the resin with (OH-) ions.Regeneration can take place off-site with exhausted resin exchangedwith deionizers (Figure 16) brought in by a service company.Regeneration can also be accomplished on-site by installing regenerable-design deionizer equipment and by proper use of thenecessary chemicals.

Figure 16 Exchange Tank Deionizer



Two-Bed and Mixed-Bed DeionizersThe two basic configurations of deionizers are two-bed and mixed-bed.

Two-bed deionizers (Figure 17) have separate tanks of cation andanion resins. In mixed-bed deionizers (Figure 18) the two resins areblended together in a single tank or vessel. Generally mixed-bed systems will produce higher-quality water, but with a lower totalcapacity than two-bed systems.

Figure 17 Two-Bed Deionizer




Figure 18 Mixed-Bed Deionizer

Deionization can produce extremely high-quality water in terms of dissolved ions or minerals, up to the maximum purity of 18.3 megohms/cm resistance. However, it generally cannot removeorganics, and can become a breeding ground for bacteria actuallydiminishing water quality if organic and microbial contamination are critical.

Failure to regenerate the resin at the proper time may result in salts remaining in the water or even worse, being increased in concentration. Even partially-exhausted resin beds can increase levels of some contaminants due to varying selectivity for ions, and may add particulates and resin fines to the deionized water.


Organic ScavengingOrganic scavengers, or traps, contain strong-base anion resin sincemost naturally-occurring organics have a slightly negative charge.After the resin is loaded the organics can be displaced by the Cl-anion during regeneration with high concentrations of sodium chloride brine.

Distillation and Pure Steam GeneratorsDistillation (Figure 19) is the collection of condensed steam produced by boiling water. Most contaminants do not vaporize and,therefore, do not pass to the condensate (also called distillate).

Figure 19 Distillation Process / Single-Effect Still Schematic


Vent

Condenser

CoolingWater

Distillate

Evaporator

High PurityChamber

Baffle

CondenserCoils

Reboiler

Inlet D.I.Feedwater

Thermosyphon

Condensate Outlet

Cooler

CondensateFeedbackPurifier

Steam HeatSupply

FloatFeeder

To Feed


With a properly-designed still, removal of both organic and inorganic contaminants, including biological impurities such aspyrogens, is attained. Since distillation involves a phase change,when properly carried out by a correctly designed and operated still,it removes all impurities down to the range of 10 parts per trillion(ppt), producing water of extremely high purity. Close control overboiling temperature and boiling rate, as well as the separation ofsteam from potential carryover, is required for the purest water.

Distillation is comparatively energy-intensive. However, the devel-opment of multiple-effect distillation (Figure 20a) has dramaticallyreduced the energy consumption required versus single-effect units(Figure 20b). Higher temperature steam is used repeatedly, losingsome heat in each stage (effect) but substantially reducing overallenergy use.

Figure 20a Multiple-Effect WFI Still



Figure 20b Single-Effect Still



Figure 20c Pure Steam Generator

Today the most significant use of stills is in laboratories and thebiotechnology and pharmaceutical industries because of their criticalconcern for biological contamination. Distillation is the most accepted technology for a consistent supply of pyrogen-free waterwithout the use of chemical additives. Careful temperature monitor-ing is required to ensure purity and avoid recontamination of thepurified water.

Membrane technologies such as reverse osmosis (RO) and ultrafil-tration (UF) are increasingly used as pretreatment to distillation toreduce maintenance caused by scaling and organic contamination,and to increase distillate quality. In most cases the RO systemremoves most organics, bacteria and pyrogens, and the majority ofthe salts. The still acts as a backup system for absolute microbe andother contaminant removal in assuring consistent USP WFI-quality(pharmaceutical) water (see Section 5.4). Some combinations of thetechnologies are unique enough to earn patent protection.




ElectrodialysisElectrodialysis (ED) and electrodialysis reversal (EDR) (Figure 21)employ electrical current and specially-prepared membranes whichare semipermeable to ions based on their charge, electrical current,and ability to reduce the ionic content of water. Two flat sheet mem-branes, one that preferentially permeates cations and the other,anions, are stacked alternately with flow channels between them.Cathode and anode electrodes are placed on each side of the alter-nating stack of membranes to draw the counter ions through themembranes, leaving lower concentrations of ions in the feedwater.

The efficiency of electrodialysis depends on the ionic solids andfouling potential from organics and particles in the feedwater, thetemperature, the flow rate, system size and required electrical current. Organics and weakly-charged inorganics are not removed by ED.Recent developments have improved the efficiency of ED by reversing the polarity of the electrodes periodically. This is calledEDR and has reduced the scaling and fouling problems common to ED.

Figure 21 Electrodialysis Reversal (EDR) System

A A ACC C

++++++

+ + ++++++

Electrode(Anode)

Feed

Electrode(Cathode)

Ion-DepletedIon-Concentrated

(Brine)

Anion Selective Membrane

Cation Selective Membrane

A

C


Crossflow Filtration Systems(Reverse Osmosis and Similar Processes)Reverse osmosis, invented in 1959, is the newest major method of water purification and one of the types of crossflow membranefiltration. It is a process which removes both dissolved organics andsalts using a mechanism different from ion exchange or activatedcarbon. The pressurized feedwater flows across a membrane, with aportion of the feed permeating the membrane. The balance of thefeed sweeps parallel to the surface of the membrane to exit the system without being filtered. The filtered stream is the permeatebecause it has permeated the membrane. The second stream is theconcentrate because it carries off the concentrated contaminantsrejected by the membrane (Figure 22). Because the feed and concen-trate flow parallel to the membrane instead of perpendicular to it, theprocess is called crossflow filtration (or, erroneously, tangentialflow).

Depending on the size of the pores engineered into the membrane,crossflow filters are effective in the classes of separation known asreverse osmosis, nanofiltration, ultrafiltration and the more recentmicrofiltration. The Filtration Spectrum (Figure 23) shows the relationship among the pore sizes and contaminants removed duringeach process.

Figure 22 Crossflow Filtration


Feed stream

Permeate

Concentrate Stream



Figure 23 Filtration Spectrum

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Crossflow membrane filtration allows continuous removal of contaminants which in normal flow filtration would blind (coverup) or plug the membrane pores very rapidly. Thus the crossflowmode of operation is essential to these processes.

Reverse Osmosis (RO)Reverse osmosis (RO) was the first crossflow membrane separationprocess to be widely commercialized. RO removes most organiccompounds and up to 99% of all ions (Figure 24). A selection of ROmembranes is available to address varying water conditions andrequirements.

Figure 24 Reverse Osmosis

RO can meet most water standards with a single-pass system and thehighest standards with a double-pass system. This process achievesrejections of 99.9+% of viruses, bacteria and pyrogens. Pressure inthe range of 50 to 1000 psig (3.4 to 69 bar) is the driving force ofthe RO purification process. It is much more energy-efficient compared to phase change processes (distillation) and more efficientthan the strong chemicals required for ion exchange regeneration.

+ ++

++

+ +

500 MW1000 MW

350 MW

200MW

100MW

300MW

200MW

50MW

300MW

Solution Flow

Pressure

Pure Water Boundary LayerMembrane SurfaceMembrane Support Layer



Nanofiltration (NF)Nanofiltration (NF) equipment removes organic compounds in the250 to 1000 molecular weight range, also rejecting some salts (typically divalent), and passing more water at lower driving pressures than RO (Figure 25). NF economically softens water without the pollution of regenerated systems and provides uniquefractionation capabilities such as organics desalting.

Figure 25 Nanofiltration

+ ++

++

+ +

+

+

+

+

+

400MW

300MW

1000 MW2000 MW

800 MW

100MW

300MW

200MW

300MW

Pressure

SolutionFlow

Pure Water Boundary LayerMembrane SurfaceMembrane Support Layer


Ultrafiltration (UF)Ultrafiltration (UF) is similar to RO and NF, but is defined as acrossflow process that does not reject ions (Figure 26). UF rejectssolutes above 1000 daltons (molecular weight). Because of the larg-er pore size in the membrane, UF requires a much lower differentialoperating pressure: 10 to 100 psig (0.7 to 6.9 bar). UF removes larger organics, colloids, bacteria, and pyrogens while allowing mostions and small organics such as sucrose to permeate the porousstructure.

Figure 26 Ultrafiltration


+ ++

++

+ +

Pressure

SolutionFlow

5000 MW20000 MW

2000 MW

1000MW

100MW

200MW

1000MW

50MW

500MW

Membrane SurfaceMembrane Support Layer


Microfiltration (MF)Microfiltration (MF) membranes are absolute filters typically ratedin the 0.1- to 3.0-micron range. Available in polymer, metal andceramic membrane discs or pleated cartridge filters, MF is now also available in crossflow configurations (Figure 27). Operating differential pressures of 5 to 25 psig (0.3 to 1.7 bar) are typical.

Figure 27 Microfiltration

Crossflow MF substantially reduces the frequency of filter mediareplacement required compared to normal flow MF because of thecontinuous self-cleaning feature. Crossflow MF systems typicallyhave a higher capital cost than MF cartridge filter systems; however,operating costs are substantially lower.


+ ++

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0.1

0.1

Pressure

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Membrane Configurations Crossflow membranes are manufactured into various configurations tubular, hollow-fiber, flat-sheet or spiral-wound. Due to relativeefficiency and economy, spiral-wound membrane elements (calledsepralators) are by far the most popular for crossflow water purification.

Sepralators (Spiral-Wound Membrane Elements)Sepralators have gained the greatest acceptance in the market. Theyare the most rugged, leak-free and pressure-resistant configuration.The spiral design allows for optimum membrane surface area andfluid dynamics to produce a high permeate flow for the size ofequipment required. Sepralators are available with RO, NF, UF, andMF membranes. Sepralators (Figure 28) are quite easy to maintainwith a routine cleaning program. A major advantage is enhancedself-cleaning due to turbulent flow at the membrane surface. Thisfeature dramatically reduces fouling, thereby enhancing performanceand membrane life. Spiral-wound designs also offer the greatestselection of membrane material, allowing users to tailor a systemdesign to suit their purification requirements.

Figure 28 Spiral-Wound Membrane Element (Sepralator)

Hollow Fine-Fiber ElementsHollow fine-fiber elements (Figure 29) consist of hollow fibers eachroughly the size of a human hair. Thousands of fibers are closelybundled in each housing. The pressurized feed flows slowly over theoutside of the fibers and pure water permeates to the center. Then

Feed Flow

Feed Flow

Permeate Flow

ConcentrateFlow

ConcentrateFlow

Permeate Tube

O-Ring

Interconnector

PermeateCarrier

Membrane

AdhesiveBond

(at edges of Membrane Envelope)

Mesh Spacer

Mesh Spacer



the water is collected out of potted tube sheet.

In the early 1970s hollow fine-fiber water purification systemsgained popularity because of their high productivity resulting fromvery high membrane surface areas. The major disadvantage of thiselement is the amount of prefiltration required to keep the tightly-packed membrane surface free of severe fouling due to the laminarflow in the element.

Figure 29 Hollow Fine-Fiber Permeator (Membrane Element)

Hollow Fat-Fiber ElementsHollow fat-fiber elements (Figure 30) are only used in UF and MFdue to burst-strength limitations. The pressurized feed flow is on theinside of the fiber and water permeates to the outside of the fiber.The fibers are potted at each end in a housing. Their self-supportingnature limits maximum feed flows. 70 psid (4.8 bar) is the pressurelimit through elements constructed with these small fibers.

Concentrate

Feed Permeate

Epoxy NubShell

Epoxy Tube Sheet

Product or Permeate End Plate

FeedEnd Plate

Hollow Fiber Membrane

Permeate Flows Inside Fibers Toward Tube Sheets

PorousSupport Block



Figure 30 Hollow Fat-Fiber Element

Disinfection Control of MicrobesControl of microorganism populations is essential in maintaining theperformance of any water system. An example is in ultrapure watersystems in which bacterial fouling is a leading cause of contamina-tion, and carefully monitored bacterial control is a necessity.

Biological control of a water system is accomplished by maintaininga continuous biocide residual throughout the system, or by sanitizingthe system on a regular basis. A continuous biocide residual ispreferable because it keeps bacterial growth in check and preventsbiofilms. However, in some high-purity water systems this is notpossible, so regular sanitizations are needed. In either case, one ofthe most effective control measures is to keep the system runningcontinuously, since bacteria reproduce more quickly during shut-down. If this is not possible, a 15- to 30-minute flush every fourhours is helpful.

Two important considerations when using a biocide are concentra-tion and contact time. The higher the concentration, the shorter thecontact time needed for effective disinfection. Other factors whichaffect biocide activity are pH, temperature, water hardness, estab-lishment of a biofilm and general cleanliness of equipment.

In many cases, the system needs to be cleaned before it is disinfect-ed. Cleaning helps to remove bacterial film and dirt that can maskbacteria and viruses in the equipment. The film would allow only the surface bacteria to be killed, and the bacteria would quickly re-establish themselves.

Module ShellPermeate

FeedConcentrate

ThermosetTube Sheet Permeate

Hollow Fiber

Feed flows outside fibers.Permeate collects in common plenum.



ChemicalOxidizing Biocides ChlorineBy far the most commonly-used biocide because of its low costand high effectiveness, chlorine is well understood, acceptedand readily available. Chlorine is most effective below pH 7.The major disadvantage is safety of handling, particularly forlarge systems which use chlorine gas.

Chlorine is dosed continually to maintain residuals of 0.2 to 2 ppm. Periodic sanitation shock treatments are accomplishedwith 100-200 ppm concentrations for 30 minutes. Care must be taken to ensure that materials of construction including membranes, filters and other items are compatible and will notbe damaged.

Chlorine GasChlorine gas is the most cost-effective form of chlorine additionfor systems over 200 gpm (757 Lpm). A special room for chlorine storage and injection is required along with substantialsafety procedures.

For smaller systems, chlorine is used in forms including sodiumhypochlorite (NaOCl) and calcium hypochlorite dihydrate[Ca(OCL)2 2H2O)] liquids. Both are available at varying concentrations.

ChloraminesChloramines are produced by reacting chlorine with ammonia.Chloramines are much more stable compared to chlorine and are used in some municipalities to ensure a residual will beavailable at the end of the distribution system. The disadvantageover chlorine is the longer contact time required by chloraminesfor disinfection.

Chlorine DioxideChlorine dioxide (ClO2) is an effective form of chlorine butbecause it is more expensive, its use is limited. It is more effective at high pH and more compatible with some mem-branes than chlorine. Another advantage is stability in storage at concentrations used for smaller systems. It can degrade aromaticcompounds such as humic and folic acids from surface watersources. It is somewhat corrosive and must be handled with care.


OzoneOzone is twice as powerful an oxidant as chlorine. Ozone (O3)is manufactured onsite by discharging an electric currentthrough air (Figures 31a & 31b). The oxygen (O2) in the airforms O3 which is highly reactive and unstable. Ozone does notadd any ionic contamination because it degrades to O2. Ozonemust be dosed into water on a continuous basis because it has a very short half-life (approximately 20 minutes at ambient temperatures) in solution. In certain applications all ozone mustbe removed prior to end use. This may be achieved by exposingthe ozonated water to ultraviolet light which breaks down theozone to oxygen.

Figure 31a Ozone Generator




Figure 31b Ozone Generator

Hydrogen PeroxideAn effective disinfectant, hydrogen peroxide (H2O2) does not addcontaminant ions to water because it degrades into H2O and O2. Thisis an advantage in critical systems such as microelectronics wherelow-level ionic contamination is a concern. Hydrogen peroxide canalso be used on membranes that cannot tolerate chlorine.

Hydrogen peroxide generally requires high concentrations to beeffective and must be catalyzed by iron or copper, which are not present in ultrapure water systems. Without a catalyst, up to a 10%(by volume) solution may be required, which is less practical.

BromineAs a halogen, bromine (Br2) is similar to chlorine in its actionsalthough the cost of bromine is greater. Bromine is used on a limitedbasis, most often for the disinfection of indoor swimming pools andspas. It maintains a residual in warm water better than chlorine, but degrades rapidly in sunlight from the ultraviolet part of the spectrum.

IodineCommonly used by campers and the military for microbial treatmentfor potable water in the field, iodine (I2) is not recommended on a



continuous basis for potable water because of its potential ill effectson human thyroid metabolism. It can be used at low concentrations(0.2 ppm) to control bacteria in RO water storage systems; however,it is approximately three times more expensive than chlorine andwill stain at higher concentrations.

Peracetic Acid A relatively new disinfectant, peracetic acid (CH3COOOH) exists inequilibrium with hydrogen peroxide and is used mainly in dialysisequipment disinfection as a replacement for formaldehyde. It isclaimed to have effectiveness similar to formaldehyde, but withoutthe handling difficulties. Also, it is compatible with some membranes which are not chlorine-tolerant, and is a small enoughmolecule to pass through the membrane and disinfect the down-stream side. It breaks down to non-hazardous acetic acid and water.Its disadvantages are high cost, toxicity in concentrated doses, instability, lack of historical effectiveness, and compatibility withmaterials of construction.

Nonoxidizing Biocides Formaldehyde (HCHO)Formaldehyde has been a commonly-used disinfectant because of its stability, effectiveness against a wide range of bacteria, and lowcorrosiveness. As a sporicide, formaldehyde can be classed as a sterilizing agent. It is being phased out of general use due to stringent government regulations on human exposure limits.

A low concentration solution, typically 0.5%, is used as a storageagent for RO and UF membranes, ion exchange resins, and storageand distribution systems. In higher concentrations, typically a 4%solution, formaldehyde is used as a shock treatment to sanitize dialysis and other hospital water-based systems. To date, a completesubstitute for formaldehyde has not been found.

Quaternary AmmoniumQuaternary ammonium compounds are most commonly used as sanitizing agents in pharmaceutical, food and medical facilities.These compounds are stable, noncorrosive, nonirritating and activeagainst a wide variety of microorganisms. Surface activity is anadvantage when cleaning is desirable.

However, quarternary ammonium compounds may cause foamingproblems in mechanical operations and form films requiring long



rinse times. Quarternary ammonium compounds are not compatiblewith some polymeric membranes.

Anionic SurfactantsAnionic surfactants have a limited biocidal activity against the kindsof bacteria (gram-negative) commonly found in pure water systems.

Physical Treatments HeatHeat is a classic form of bacterial control and is very effective whensystems are properly designed and installed. Temperatures of 80C(176F) are commonly used in pharmaceutical facilities for storageand recirculation of USP purified water and WFI. Heat treatmentabove 80C (176F) is also used to control microorganisms in activated carbon systems.

Ultraviolet Light (UV)Treatment with ultraviolet light is a popular form of disinfection dueto ease of use. Water is exposed at a controlled rate to ultravioletlight waves. The light deactivates DNA leading to bacterial reduction. With proper design and maintenance, UV systems aresimple and reliable for a high reduction in bacteria (99+ %), and are compatible with chemically-sensitive membrane and DI systemswhich are often incompatible with chemicals.

UV is used to reduce microbial loading to membrane systems and to maintain low bacterial counts in high-purity water storage andrecirculation systems. If ozone has been added to water, UV is effective in destroying ozone residuals prior to end use. UV willincrease the conductivity of water when organics are in the solutiondue to the breakdown of the organics and formation of weak organicacid.

The disadvantage of UV light is lack of an active residual, and it iseffective only if there is direct UV light contact with the microbes.Careful system design and operation is required to ensure bacterialreduction. Inadequate light may only damage bacteria, which canrecover. The water must be free of suspended solids that can shadow bacteria from adequate UV contact.

Point-of-Use MicrofiltrationMost bacteria have physical diameters in excess of 0.2 micron. Thus, a 0.2-micron or smaller-rated filter will mechanically remove



bacteria continuously from a flowing system. Point-of-use microfiltration is commonly used in pharmaceutical, medical, andmicroelectronics applications as assurance against bacterial contamination. To be used as a sterilizing filter, filters must beabsolute rated (i.e., complete retention of particles equal to or largerthan the filter micron rating). For pharmaceutical and medical applications these filters must undergo validation by means of a rigorous bacteria challenge test. Individual filters must be integrity-tested when in place in the system to ensure that the filter is properlysealed and defect-free. The greatest advantage of microfiltration isthat neither chemicals nor heat is required. Filters must be changedon a regular basis to prevent the possibility of grow-through or pressure breakthrough.


5.0 EXAMPLES OF HIGH-PURITY WATER TREATMENTSYSTEMS

Each water purification situation is different. Feedwater composition variesas widely as purification requirements. However, some general hardware configurations are described here which have proven both efficient and cost-effective for common applications.

Feedwater and/or product water specifications may vary substantially from those described here, possibly requiring additional or alternative treatment methods. A water treatment professional should be consulted before designing a new water treatment system or modifying an existing system.

5.1 Potable Water

Residential Water Purification System With the growing awareness of water quality concerns among thegeneral public, many homeowners are installing under-the-sink orpoint-of-entry water purification systems to augment municipaltreatment and/or their home water softener or iron filter.

The most complete system would use reverse osmosis to reduceTDS by approximately 90%, activated carbon to adsorb small molecular weight organics and chlorine, and final submicron filtration to remove carbon fines, other particles and bacteria whichmay grow in the carbon filter.

Most municipal water supplies in North America meet or exceed the World Health Organization (WHO) standard for potable water.However, several possible areas of concern exist, such as THMs,hydrocarbon compounds, and heavy metals. Within the residence,contamination from lead solder in the pipes may also be a concern.

Product water: up to 1 gpm (3.8 Lpm) on demand; removal of 90% of lead, aluminum, and hydrocarbon compounds (Figure 32).

HIGH-PURITY WATER TREATMENT SYSTEMS


Figure 32 Home RO

Typical system used to meet standards. Other modifications are dependent upon concentration of feed, quality of water required, and other objectives.


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5.2 Kidney Dialysis

The suggested limits and treatment methods outlined are based onstandards published by the Association for the Advancement ofMedical Instrumentation (AAMI) and the American Society forArtificial Internal Organs (ASAIO). The methods selected are alsobased on AAMI recommendations as contained in the handbookAmerican National Standards, Hemodialysis Systems. The medical concern is to eliminate hemolysis in the blood, and thepotential for pyrogenic reactions. See Appendix for AAMI waterstandards.

Single-Patient Dialysis Specifications: 12 gph (345 Lph) requirement at 20 psi (1.4 bar)

pressure required Feedwater: 400 ppm TDS; 2.0 mg/L chlorine 77F (25C)

Pre-TreatmentActivated Carbon FiltrationTen-inch Filter Housing with 5-micron Blown Microfiber Prefilter

Reverse Osmosis UnitPermeate Capacity: 14.5 gph (55 Lph) at 77F (25C)Recovery: 33%

Options to ConsiderChlorine Test KitPortable Conductivity MeterWater Softener



Figure 33 Single-Patient Dialysis

Typical system used to meet standards. Other modifications are dependent upon concentration of feed, quality of water required, and other objectives.


Poin

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Wat

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Car

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Filte

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60 g

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Perm

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Skid

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Car

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120

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15-Bed, In-Center Dialysis System, with Recycle Specifications: 3.0 gpm (11.5 Lpm) requirement; some

storage required with continuous recycle to storage System designed to meet AAMI Standards Feedwater: 400 ppm TDS; 2.0 mg/L free chlorine; 13.9 gpg

hardness; 77F (25C)

Pre-TreatmentWater Softener (24 hours of operation between regenerations)Activated Carbon Filter

Reverse Osmosis UnitPermeate Capacity: 188 gph (712 Lph) at 77F (25C)

Storage and DistributionStor

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